In conventional electrophotography, it is known to imagewise apply toner particles in piles on a receiver to form a toner image. The toner image is then fused to form a permanent image that is bound to the receiver. In color electrophotography, fusing is also used to enable two or more colors of toner to mix to form a combination color. Accordingly, proper fusing of electrophotographic toner is essential to the formation of high quality electrophotographic images.
While other types of fusing exist, such as those that involve the use of solvents or pressure differentials, fusing is typically achieved by heating the toner in a toner image to a temperature that is higher than a glass transition temperature of the toner. There are several key variables that impact the effectiveness of such thermal toner fusing. These include the rate at which energy can be supplied by all sources to heat the toner during fusing, the amount of exposure time during which the energy can be applied for the purpose of fusing, the rate at which the energy can be absorbed and transferred by a given unit of the thickness of toner without causing damage to the toner, the toner piles formed on the receiver, and the amount of ambient pressure applied to the toner pile during exposure.
These variables can be combined in a variety of ways to achieve a fusing solution. One variable that is generally held fixed in determining a fusing solution is the stack height of the toner pile on the receiver. Typically, the stack height is controlled to be within a predefined range. This reduces the cost of images printed using the toner and also reduces the number of variables that must be managed when determining a fusing solution. Further, the use of relatively consistent toner pile thickness across a toner image allows all of the other fusing variables to be set once and maintained at a steady state. Typically toner stack heights are maintained in a range of less than about 20 μm.
Various conventional technologies are known that are adapted to thermally fused toner piles that have such managed stack heights. In one example of contact fusing, known as hot roller fusing, a receiver having a toner image applied thereto is passed between a nip and a heated roller or belt. Heat and pressure are applied to the toner image and receiver causing the toner to heat to a temperature at or above the glass transition temperature of the toner. U.S. Pat. No. 6,577,840, entitled “Method and Apparatus for Image Forming Capable of Effectively Performing an Image Fixing Process”, issued to Hachisuka et al. on Jun. 10, 2003 shows one example of a heated roller type fuser while U.S. Pat. No. 7,630,677, entitled “Image Heating Apparatus”, issued to Osada et al. on Dec. 8, 2009 shows one example of heated belt fuser.
Similarly, various forms of non-contact fusing are known that can cause a toner to be heated. U.S. Pat. No. 7,630,674 entitled “Method and Arrangement for Fusing Toner Images to a Printing Material” shows one example of this.
Combinations of contact fusing and non-contact fusing are also known. For example, U.S. Pat. No. 6,909,871 entitled “Method and Device for Fusing Toner Onto a Substrate” shows a combination of microwave and pressure roller heating to achieve a fusing solution to allow fusing to occur in during abbreviated exposure times in order to enable high rates of printing.
Recently, it has become popular to provide toner images having portions with high toner stack heights such as those that include for example and without limitation stack heights that are on the order of 50 μm to 500 μm. An advantage of such high toner stack heights is that they can be used to form projections from a surface of an image that can impart a three dimensional look and/or feel to an image. This extra dimension, is provided by a contrast in toner stack heights which can range from a conventional stack height to, as noted above, stack heights of up to 500 μm.
Conventional fusing technologies however are not easily applied to the purpose of fusing toner images having toner piles that have high toner stack heights. In part, this is because the rate at which thermal energy can be transferred to and into a unit of toner is such that only a conventional toner pile thickness can be fully fused during a fusing operation that is performed at desirable and commercially profitable commercial printing speeds. In part this is also because of the extent of the variability in toner stack heights within the toner image.
This problem is not easily solved in general and in particular where fusing is to be performed at production speeds. If insufficient energy is applied during the short time periods allotted for fusing at high production speeds, incomplete fusing can occur. Incomplete fusing can cause mechanical defects to arise in the printed images such as incomplete bonding of the toner pile to the receiver. This can lead to full or partial separation of the toner pile from the receiver resulting in an unacceptable image. Similarly, incomplete fusing can introduce weaknesses in the resultant toner pile such as pockets of unfused dry toner that can cause fracture of the toner itself, color mixing problems, gloss variations or partial separation of the toner powder from the receiver.
However, markedly increasing the amount of energy applied during a fusing step creates other problems in image formation. For example, as is described in commonly assigned U.S. Pat. Pub. No. 2009/014948 entitled “Enhanced Fuser Offset Latitude Method” filed by Cahill et al., on Dec. 18, 2007 using high temperatures for example on a roller type fuser can cause image artifacts. Such artifacts occur when toner that is in contact with a hot roller transitions to a glass transition temperature of the toner before toner that is closer to the receiver makes this transition. This can cause a portion of the toner to adhere to and contaminate the heated roller or other rollers associated with a fuser and can cause a variety of unwanted artifacts in an image. Similarly, as noted in the '671 patent, in non-contact fusing such as microwave increased energy can create artifacts such as blister formation of the toner on the receiver.
For these reasons, a fusing solution must be managed so that sufficient energy is transferred to a toner during a fusing process to allow fusing to occur and so that the artifacts created by applying too much energy during a short period of are not created.
It will be appreciated that reaching such a solution is made more difficult by the increased energy load that must be delivered to heat a thick toner pile to ensure full fusing during the short fusing process allowed during printing. It will also be appreciated that there are inherent limitations on the rate at which toner can transfer energy through a toner pile without creating the aforementioned hot offset problems.
What is needed is a system that can thoroughly fuse toner images having toner piles with toner stack heights that are greater than about 50 μm.